The present invention relates to a method for joining a plurality of superposed metal sheets by local plastic deformation so that an interengaging connection is formed.
A typical method of this kind is disclosed in U.S. Pat. No. 4,601,090, particularly in FIGS. 8, 9 and 11 through 15. A predetermined volume of sheet material is displaced, by means of appropriate dies, in a direction generally orthogonal to the sheet surfaces. The protuberance so formed is then squeezed causing the metal material to cold-flow laterally thereby forming the interengaging connection. The joining element so produced remains projected beyond the sheet surface. Such a projection is often considered to be a disadvantage both functionally and aesthetically.
For this reason attempts have been made to flatten such joints, as disclosed in European Patent Application 282 902 which also discloses some tools suited for this kind of joining technique.
Therefore, it is an object of the present invention to provide a method of the general nature set forth above wherein the aspect of the joint is improved in that the interengagement of the sheets occurs within the volume delimited by the outer sheet surfaces.